1. Field of the Invention
This invention relates to a process of producing briquettes to be charged to zinc-producing shaft furnaces, in which process calcined material, which has been obtained by roasting in a fluidized bed and contains zinc oxide is subjected to hot briquetting.
2. Discussion of Prior Art
For the production of zinc in a shaft furnace, the sulfidic ore concentrates must be roasted and formed into lumps. Roasting by sintering on a traveling grate is rather expensive and the resulting exhaust gas has a relatively low SO.sub.2 content. Additionally, that operation gives rise to problems relating to pollution of the environment. These problems can be avoided to a large extent by a roasting in a fluidized bed. However, the resulting fine-grained calcined material, hereinafter referred to as a "calcine", must be formed into lumps in a separate process step.
It is known from British Pat. No. 1,302,864 to form calcine obtained by roasting in a fluidized bed into briquettes at temperatures of at least 500.degree. C. and under a pressure of 0.157 to 3.15 metric tons per cm.sup.2 without addition of carbonaceous binders. However, the resulting material can be reduced much less easily than sintered material.
German 23 60 346 discloses the production of briquettes which incorporate bonded fine-grained coke. However, only fine-grained zinc oxide and, if desired, fine-grained lead oxide, which may have been obtained by the Waelz process, can be processed in that manner. That pulverulent material must first be formed into pellets, which are 2 to 10 mm in diameter and subsequently briquetted at 500.degree. to 800.degree. C. Pelletizing constitutes an additional operation and the pellets must be dried before they are briquetted because the briquettes would otherwise burst. This drying is also required since moisture is not desired in the zinc-producing shaft furnace as it tends to re-oxidize the zinc vapor.
From V. Tafel, Lehrbuch der Metallhuttenkunde 1953, Volume II, pages 518-519, it is known to obtain charge material for a recovery of zinc in a vertical retort furnace in that calcine is briquetted together with caking coal and a binder and to coke the briquettes before they are charged to the retort furnaces. It is also known to briquette ores or oxides together with caking coal or bituminous binders, which are heated to a plastic consistency (German 12 52 623; German Offenlegungsschrift 23 35 669; German 718,967; U.S. Pat. No. 3,212,877). That process requires a separate coking step and expulsion of volatile constituents may result in cracking and bursting of the briquettes.
It is an object of the invention to provide an inexpensive process for the treatment of calcine obtained by roasting in a fluidized bed so as to form briquettes which have desirable properties for the production of zinc in a shaft furnace.